common/rc: factor out _scratch_xfs_[get|set]_sb_field

common/rc: factor out _scratch_xfs_[get|set]_sb_field

It's common to get and set the values of fields in XFS super block,
so factor them out as scratch_xfs_[get|set]_sb_field, reimplement
them based on _scratch_xfs_[get|set]_metadata_field, and update the
related test cases accordingly.

Also move _scratch_xfs_[get|set]_metadata_field from common/fuzzy to
common/xfs.

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>